Mooring Disconnect Arrangement

ABSTRACT

A mooring disconnect arrangement for a floating offshore structure and a mooring buoy. At least one high pressure water jet is positioned to direct water between the floating offshore structure and the mooring buoy. The floating offshore structure and mooring buoy are also provided with specially shaped complementary surfaces to assist in disconnection and separation. One or more mechanical restraining devices may be used to retain the floating offshore structure and mooring buoy connected together during normal drilling or production operations.

FIELD AND BACKGROUND OF INVENTION

The invention is generally related to the disconnection of mooredfloating offshore structures from mooring buoys while underenvironmental loads.

In the offshore industry of drilling for and producing oil and naturalgas, bottom founded and floating moored structures are used. There aretimes when floating structures that are moored in place must be releasedfrom their moorings and moved due to high environmental forces such assea ice or storms such as hurricanes.

When it becomes necessary to disconnect a floating structure due to theeminent danger of high environmental forces it is preferable that themooring arrangements remain intact for reattachment at a later time.

Current existing and proposed arrangements for disconnecting a floatingstructure from their mooring arrangements are based on two basicapproaches.

One approach is to disconnect each mooring line individually. This canresult in the individual lines becoming entangled. The mooringarrangement must then be recovered and reinstalled line-by-line. Therisks associated with this approach are 1) recoil of the mooring lineswhen released under tension striking and causing damage to the floatingstructure and 2) being very time consuming to recover each line andreconnect individually. This approach is especially not practical in icecovered waters.

A second approach is the use of a buoy that supports a mooring spreadand keeps all lines attached to the disconnected buoy for subsequentreconnection as a group. This approach is typically based on a conicalshaped buoy arrangement that drops away from the floating structure andfacilitates all of the mooring lines remaining connected to thedisconnected buoy, This keeps the lines together in a group, as comparedto individual line disconnect, but the height of the buoy being releasedis constrained by the beam dimension of the floating vessel beingreleased. This can result in a longer time duration for the buoy toclear the disconnected floating structure.

The objectives of mooring systems that may he disconnected for thispurpose are a quick release and quickly increasing the distance betweenthe mooring and the floating offshore structure.

In general, the risks or problems that occur with these disconnectablearrangements are 1) binding between the mooring and floating structurewhen the mechanism is expected to release and 2) contact interactionbetween the mooring and floating structure that results in damage to oneor both structures after the mooring system is released.

Binding may occur because the surfaces that are supposed to separatehave been in contact for a number of years prior to the first disconnectattempt. The two bodies can he forced apart by mechanical devices, butthese devices must be released as soon as separation occurs to preventdamage to the releasing device. Another major risk is that, due to thetwo bodies moving independently after release, but still in the sameproximity, they can collide causing damage to one or both structures.

When the release mechanism is based on a conical buoy supporting themoorings, lower profiles of the buoy facilitate a quicker release andclearance growth. The load from the floating structure is transferred tothe mooring arrangement through the contact area between the floatingstructure and the buoy. For a given design load, this area will be thesame for any buoy shape. This area can be developed by making thedisconnectable buoy high with a relatively small diameter (FIG. 1A) orby making the height low with a larger diameter (FIG. 1B). Assuming thesame rate of vertical separation, the buoy with the lower profile willseparate more quickly than the higher buoy, thus reducing the risk ofinteraction after separation.

Therefore, it can be seen there is a need for an improved means ofreleasing a floating offshore structure from its mooring while underenvironmental loads.

SUMMARY OF INVENTION

The present invention addresses the above need and is drawn to a mooringdisconnect arrangement for a floating offshore structure and a mooringbuoy. At least one high pressure water jet is positioned to direct waterbetween the floating offshore structure and the mooring buoy. Thefloating offshore structure and mooring buoy are also provided withspecially shaped complementary surfaces to assist in disconnection andseparation. One or more mechanical restraining devices may be used toretain the floating offshore structure and mooring buoy connectedtogether during normal drilling or production operations.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming partof this disclosure. For a better understanding of the present invention,and the operating advantages attained by its use, reference is made tothe accompanying drawings and descriptive matter, forming a part of thisdisclosure, in which a preferred embodiment of the invention isillustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, forming a part of this specification, andin which reference numerals shown in the drawings designate like orcorresponding parts throughout the same:

FIG. 1A illustrates a prior art high profile release buoy.

FIG. 1B illustrates a prior art low profile release buoy,

FIG. 2-5 schematically illustrate the invention and the separation ofthe floating structure from the mooring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 2, the mooring disconnect arrangement 10 comprises theuse of high pressure water jets 12 in conjunction with specially shapedinterface surfaces 14 on the floating offshore structure 16 and themooring buoy 18.

It should be understood that only the lower portion of the floatingoffshore structure 16 is shown which is normally well below the surfaceof the water. Thus, the water surface is not shown in relation to theinvention and it is to be understood that the connection anddisconnection sequences happen below the water surface.

The water jets 12 are preferably provided on the hull of the floatingoffshore structure 16. The water capacity and pressure, number of waterjets 12, and spacing between the water jets 12 is determined by the sizeof the floating offshore structure 16 and mooring buoy 18.

During normal offshore operations of drilling for or producing oil andnatural gas, mechanical connection or restraining devices 20 are used torestrain the floating offshore structure 16 and mooring buoy 18 lockedtogether. Any suitable connection devices such as hydraulic rams or rackand pinion jacking arrangements may be used as the mechanical connectiondevices 20.

Ballasting force may also be used to force the floating offshorestructure and mooring buoy together.

A downward facing cone shape 22 on the floating offshore structure 16 ispreferably used in conjunction with a complementary shape 24 on themooring buoy 18 for receiving the cone shape 22. As seen in thedrawings, the cone shape 22 and its complementary shaped receiver 24 arepreferably designed to have a low profile height that is less thancurrently existing designs.

In operation, the floating offshore structure 16 and mooring buoy 18 areheld together during drilling or production operations by mechanicalrestraining devices 20. When environmental forces cause the need todisconnect, the mechanical restraining devices 20 are released and highpressure water jets 12 are activated to help initiate separation of thefloating offshore structure 16 from the mooring buoy 18.

As seen in FIG. 3, the mooring lines 26 aid in retaining a force forcausing the mooring buoy 18 to return to its normal equilibriumposition.

FIG. 4 illustrates a scenario where the force of ice may cause theoffshore floating structure 16 to rotate prior to full disconnection andseparation from the mooring buoy 18. However, it can be seen that thespecially shaped, low profile surfaces aid in separation.

As illustrated in FIG. 5, ballasting forces may also be used to aid inseparation of the floating offshore structure 16 from the mooring buoy18 by removing ballast from the floating offshore structure 16 (causingit to float upward) and adding ballast to the mooring buoy 18 (causingit to move downward). The illustrated change in normal trim angle of themooring buoy 18 is caused by the ballasting forces and pressure from thewater jets 12.

The invention provides several advantages over the previously used meansof disconnecting the mooring system.

It allows a floating structure to be disconnected from the mooring whileunder environmental loads, such as sea ice, and the upper section of thefloating structure to be removed from the continued threat while keepingthe mooring arrangement intact for reattachment to the floatingstructure.

The invention solves the problem of binding and maintains a safedistance between the floating structure and the buoy supporting themooring arrangement.

The invention facilitates a quicker release than the prior art and thusreduces the risk of damage to the offshore structure and mooring due tocontact during the release.

The method of release of the invention helps to prevent binding betweenthe buoy carrying the mooring lines and the floating structure.

While specific embodiments and/or details of the invention have beenshown and described above to illustrate the application of theprinciples of the invention, it is understood that this invention may beembodied as more fully described in the claims, or as otherwise known bythose skilled in the art (including any and all equivalents), withoutdeparting from such principles.

1. A mooring disconnect arrangement for a floating offshore structureand a mooring buoy, comprising: a. at least one high pressure water jetfor directing water between the floating offshore structure and mooringbuoy during the disconnection operation; and b. complementary lowprofile interface surfaces on the floating offshore structure andmooring buoy that aid in separation.
 2. The mooring disconnectarrangement of claim 1, wherein the interface surface on the floatingoffshore structure is a cone shape.
 3. The mooring disconnectarrangement of claim 2, wherein the interface surface on the floatingoffshore structure is an inverted cone shape.
 4. A mooring disconnectarrangement for a floating offshore structure and a mooring buoy,comprising: a. at least one high pressure water jet for directing waterbetween the floating offshore structure and mooring buoy during thedisconnection operation; b. complementary low profile interface surfaceson the floating offshore structure and mooring buoy that aid inseparation; and c. releasable mechanical restraining means that lock thefloating offshore structure and mooring buoy together during normaldrilling and production operations.
 5. The mooring disconnectarrangement of claim 4, wherein the interface surface on the floatingoffshore structure is a cone shape.
 6. The mooring disconnectarrangement of claim 5, wherein the interface surface on the floatingsurface is an inverted cone shape.
 7. A mooring disconnect arrangementfor a floating offshore structure and a mooring buoy, comprising: a. atleast one high pressure water jet for directing water between thefloating offshore structure and mooring buoy during the disconnectionoperation; b, complementary low profile interface surfaces on thefloating offshore structure and mooring buoy that aid in separation,with the interface surface on the floating offshore structure being aninverted cone shape; and c. releasable mechanical restraining means thatlock the floating offshore structure and mooring buoy together duringnormal drilling and production operations.